Severity of bleeding in persons with factor IX deficiency is generally proportional to the factor IX activity. At the Mountain States Regional Hemophilia Center-Denver (MCHSC-D) there is a family with bleeding out of proportion to the severity of their factor IX deficiency. We choose to study their factor IX proteins to assess this discrepancy. Understanding the molecular function of factor IX will lead to a more thorough comprehension of normal and pathologic hemorrhage.